Electrocatalytic Reactions for Converting CO2 to Value-Added Products: Recent Progress and Emerging Trends

Carbon dioxide (CO2) emissions are an important environmental issue that causes greenhouse and climate change effects on the earth. Nowadays, CO2 has various conversion methods to be a potential carbon resource, such as photocatalytic, electrocatalytic, and photo-electrocatalytic. CO2 conversion into value-added products has many advantages, including facile control of the reaction rate by adjusting the applied voltage and minimal environmental pollution. The development of efficient electrocatalysts and improving their viability with appropriate reactor designs is essential for the commercialization of this environmentally friendly method. In addition, microbial electrosynthesis which utilizes an electroactive bio-film electrode as a catalyst can be considered as another option to reduce CO2. This review highlights the methods which can contribute to the increase in efficiency of carbon dioxide reduction (CO2R) processes through electrode structure with the introduction of various electrolytes such as ionic liquid, sulfate, and bicarbonate electrolytes, with the control of pH and with the control of the operating pressure and temperature of the electrolyzer. It also presents the research status, a fundamental understanding of carbon dioxide reduction reaction (CO2RR) mechanisms, the development of electrochemical CO2R technologies, and challenges and opportunities for future research.


Introduction
Since the industrial revolution of the 19th century, fossil fuels such as petroleum, natural gas, and coal have been used as the main source of energy to power economies and civilizations [1]. There is a need to reduce CO 2 emissions because the burning of these fossil fuels has resulted in excessive CO 2 emissions into the atmosphere, which have had significant negative effects on the environment and pose an immediate threat to human societies [2][3][4]. The swift transformation of the need of energy and chemical industries from fossil fuels to renewable energy resources, for example, solar and wind, can be identified as one of the solutions to achieve the closed-looped configurations on the carbon footprint [5][6][7].
Nonetheless, several artificial solutions to limit or reduce CO 2 emissions have been created, such as technological innovation to increase coal burning efficiency in boilers (reducing coal consumption) and carbon capture and sequestration (CCS) [8][9][10] though CCS is a costly and an energy-consuming technology. In fact, dangerous CO 2 leakage is a mote catalyst development [49,50]. Microbial electrosynthesis (MES) utilizes self-replicating bacteria as a catalyst at room temperature and pressure, which enables a more economical and ecologically benign process than traditional chemical catalyst-based conversion. To metabolize CO 2 , bacteria in MES exchange electrons directly or indirectly using electron shuttle molecules [54]. To recycle anthropogenic CO 2 , electroactive microorganisms are employed in MES as a biocatalyst on suitable electrode materials [55].
Therefore, the current review covers a brief background of the efforts and strategies undertaken in the scientific community to improve electrocatalytic reactions for converting carbon dioxide into value-added products in terms of electrocatalyst materials and their morphology, electrolyte, temperature, pressure, and applied voltages. Furthermore, the newest accomplishments of microbial electrosynthesis for CO 2 conversion are discussed. In addition, some intriguing reaction mechanisms linked to electrocatalysts are described and discussed. Finally, in this fascinating area of research, potential future difficulties and outlooks of electrocatalytic CO 2 conversion into value-added products are proposed.

Concepts of Electrochemical CO 2 Reduction Reaction
The electrochemical conversion of CO 2 , a linear stable molecule with a powerful C-O bond (750 kJ mol −1 ), is challenging. Multi-electron/proton transfer processes, a large variety of possible reaction intermediates, and an ECR in an aqueous electrolyte are all part of the extremely complicated process of ECR [56,57].
Electrochemical reduction has been researched in aqueous solutions with various metal cathodes, as well as in several organic solvents. Although the successfully documented six-electron and eight-electron conversions to methanol and methane exist, the commonly discussed reduction products are carbon monoxide, acetic acid, and formic acid [58][59][60]. The main ECR products' half electrochemical thermodynamic reactions are shown in Table 1, ethanol (CH 3 CH 2 OH), ethylene (C 2 H 4 ), formic acid (HCOOH), methanol (CH 3 OH), methane (CH 4 ), carbon monoxide (CO), and acetate (CH 3 COOH), with reporting of their standard redox potentials at acid and base electrolytes [61,62]. In an ECR process, CO 2 molecules adsorb on the catalyst surface and interact with the atoms there to produce *CO 2 , which is then followed by many progressive transfers of electrons and/or protons toward different end products. For instance, methane is thought to originate via the pathways given below (Scheme 1) [63]: electrons and/or protons toward different end products. For instance, methane is thought to originate via the pathways given below (Scheme 1) [63]: Scheme 1. Pathway for the electrochemical conversion of methane from CO2.
A multistep reaction process, electrochemical CO2 reduction typically involves a different number of electron reaction pathways. The reaction frequently happens at the electrolyte-electrode interface for heterogeneous catalysts used in CO2 reduction, where the electrode is typically a solid electrocatalyst and the electrolyte is typically an aqueous solution saturated with CO2 through bubbling.
Water is transformed to oxygen and CO2 is reduced to the CO2 anion radical at the anode in a single-electron ECR (CO2 − ). The first step of converting CO2 to reduced carbon species is difficult because the reaction rate is very slow. The single-electron CO2 reduction to CO2 − with a pH of 7 exhibits an unfavorable and energetic reaction, with a thermodynamic potential of roughly −1.90 V vs. SHE. Furthermore, the formation of the CO2 intermediate is essential to the formation of the 2e -reduction products and the initial process can be considered the rate-limiting step [64].
Several electron/proton transfer processes are involved in the electrochemical CO2RR, and CO2 can be reduced into a collection of gaseous and liquid products by diverse pathways, including hydrocarbons (CH4 and C2H4), alcohols (CH3OH and C2H5OH), carbon monoxide (CO), and formic acid (HCOOH) [65]. This depends on the electrolytic conditions and the electrocatalysts used (e.g., applied potential, electrolyte, etc.) [28,37,66]. Without a catalyst, it is challenging to complete the first stage of CO2 activation, which produces the intermediate CO2 − radical. However, with the aid of an electrocatalyst, the CO2 − radical can be stabilized via a chemical link created between CO2 and the electrocatalyst, leading to less negative redox potential. Moreover, proton-coupled electron transfer is advantageous at the likely range of 0.20 to 0.60 V vs. SHE. The end products are influenced by the electrocatalyst and electrolyte selections as well as the quantities of electrons and protons transferred [51]. Therefore, the activation routes of some typical products in CO2RR are briefly shown in Figure 1 [64].  A multistep reaction process, electrochemical CO 2 reduction typically involves a different number of electron reaction pathways. The reaction frequently happens at the electrolyte-electrode interface for heterogeneous catalysts used in CO 2 reduction, where the electrode is typically a solid electrocatalyst and the electrolyte is typically an aqueous solution saturated with CO 2 through bubbling.
Water is transformed to oxygen and CO 2 is reduced to the CO 2 anion radical at the anode in a single-electron ECR (CO 2 − ). The first step of converting CO 2 to reduced carbon species is difficult because the reaction rate is very slow. The single-electron CO 2 reduction to CO 2 − with a pH of 7 exhibits an unfavorable and energetic reaction, with a thermodynamic potential of roughly −1.90 V vs. SHE. Furthermore, the formation of the CO 2 intermediate is essential to the formation of the 2e − reduction products and the initial process can be considered the rate-limiting step [64].
Several electron/proton transfer processes are involved in the electrochemical CO 2 RR, and CO 2 can be reduced into a collection of gaseous and liquid products by diverse pathways, including hydrocarbons (CH 4 and C 2 H 4 ), alcohols (CH 3 OH and C 2 H 5 OH), carbon monoxide (CO), and formic acid (HCOOH) [65]. This depends on the electrolytic conditions and the electrocatalysts used (e.g., applied potential, electrolyte, etc.) [28,37,66]. Without a catalyst, it is challenging to complete the first stage of CO 2 activation, which produces the intermediate CO 2 − radical. However, with the aid of an electrocatalyst, the CO 2 − radical can be stabilized via a chemical link created between CO 2 and the electrocatalyst, leading to less negative redox potential. Moreover, proton-coupled electron transfer is advantageous at the likely range of 0.20 to 0.60 V vs. SHE. The end products are influenced by the electrocatalyst and electrolyte selections as well as the quantities of electrons and protons transferred [51]. Therefore, the activation routes of some typical products in CO 2 RR are briefly shown in Figure 1 [64]. A multistep reaction process, electrochemical CO2 reduction typically involves a different number of electron reaction pathways. The reaction frequently happens at the electrolyte-electrode interface for heterogeneous catalysts used in CO2 reduction, where the electrode is typically a solid electrocatalyst and the electrolyte is typically an aqueous solution saturated with CO2 through bubbling.
Water is transformed to oxygen and CO2 is reduced to the CO2 anion radical at the anode in a single-electron ECR (CO2 − ). The first step of converting CO2 to reduced carbon species is difficult because the reaction rate is very slow. The single-electron CO2 reduction to CO2 − with a pH of 7 exhibits an unfavorable and energetic reaction, with a thermodynamic potential of roughly −1.90 V vs. SHE. Furthermore, the formation of the CO2 intermediate is essential to the formation of the 2e -reduction products and the initial process can be considered the rate-limiting step [64].
Several electron/proton transfer processes are involved in the electrochemical CO2RR, and CO2 can be reduced into a collection of gaseous and liquid products by diverse pathways, including hydrocarbons (CH4 and C2H4), alcohols (CH3OH and C2H5OH), carbon monoxide (CO), and formic acid (HCOOH) [65]. This depends on the electrolytic conditions and the electrocatalysts used (e.g., applied potential, electrolyte, etc.) [28,37,66]. Without a catalyst, it is challenging to complete the first stage of CO2 activation, which produces the intermediate CO2 − radical. However, with the aid of an electrocatalyst, the CO2 − radical can be stabilized via a chemical link created between CO2 and the electrocatalyst, leading to less negative redox potential. Moreover, proton-coupled electron transfer is advantageous at the likely range of 0.20 to 0.60 V vs. SHE. The end products are influenced by the electrocatalyst and electrolyte selections as well as the quantities of electrons and protons transferred [51]. Therefore, the activation routes of some typical products in CO2RR are briefly shown in Figure 1    Molecule reactants may react with various CO 2 RR intermediates at any phase since CO 2 RR contains several reaction steps and intermediates, which greatly broadens the range of possible products. Consequently, potential products can be selectively derived through the adjustment of the adsorption and desorption capability of electrocatalysts to distinct reaction intermediates from coupled CO 2 RR [64].
A laboratory electrochemical H-cell consists of oxygen evolution reaction (OER) happening at the surface of an anode that generates electrons (e − ) and protons (H + ) or consumes hydroxyl ions (OH − ); a cathode in order to reduce CO 2 to produces such as HCOOH/HCOO − or CO, and make OH − ; an electrolyte with the intention of transporting CO 2 to the active cathode sites and conduct ions; a membrane that allows ion exchange to take apart the anode and cathode; and a bias with suitable value to move electrons from anode to cathode (Figure 2a). A few crucial steps in a CO 2 R process are involved in such a system, including (1) movement of products into liquid phases or bulk gases from the cathode/electrolyte interface, (2) product desorption from the electrode, (3) transfer of electrons from the cathode to intermediates, (4) adsorption of CO 2 into adsorbed intermediates such as *CHO, *CO, and *COOH, (5) the surface of the cathode absorbing CO 2 , (6) transport of dissolved CO 2 to the cathode/electrolyte interface from the bulk electrolyte, and (7) CO 2 mass transfer to the bulk electrolyte from the gas phase [67]. Molecule reactants may react with various CO2RR intermediates at any phase since CO2RR contains several reaction steps and intermediates, which greatly broadens the range of possible products. Consequently, potential products can be selectively derived through the adjustment of the adsorption and desorption capability of electrocatalysts to distinct reaction intermediates from coupled CO2RR [64].
A laboratory electrochemical H-cell consists of oxygen evolution reaction (OER) happening at the surface of an anode that generates electrons (e − ) and protons (H + ) or consumes hydroxyl ions (OH − ); a cathode in order to reduce CO2 to produces such as HCOOH/HCOO − or CO, and make OH − ; an electrolyte with the intention of transporting CO2 to the active cathode sites and conduct ions; a membrane that allows ion exchange to take apart the anode and cathode; and a bias with suitable value to move electrons from anode to cathode (Figure 2a). A few crucial steps in a CO2R process are involved in such a system, including (1) movement of products into liquid phases or bulk gases from the cathode/electrolyte interface, (2) product desorption from the electrode, (3) transfer of electrons from the cathode to intermediates, (4) adsorption of CO2 into adsorbed intermediates such as *CHO, *CO, and *COOH, (5) the surface of the cathode absorbing CO2, (6) transport of dissolved CO2 to the cathode/electrolyte interface from the bulk electrolyte, and (7) CO2 mass transfer to the bulk electrolyte from the gas phase [67].  [67]. (b) At 25 C and 1 atm, standard equilibrium potentials for the half-cell hydrogen evolution and other CO2 reduction reactions [36].
One of the most critical problems of the electrochemical CO2R technologies to function at large-scale is to obtain a great CO2 selectivity to desired value-added products to reduce product separation costs and complexity. High selectivity is difficult to achieve due to, as shown in Figure 2b, the majority of CO2R reactions' standard potentials (Eo) and the hydrogen evolution reaction (HER) all being within a limited variety (−0.250 V to 0.169 V vs. standard hydrogen electrode) (SHE) [36].

Product Selectivity Parameters
The applied potential, pressure, temperature, type of electrolyte (pH, concentration, and composition), and type of electrocatalyst (crystallographic structure, chemical state, composition, and morphology) are all variables that affect selectivity, FE, and ECR performance.
In addition, the selectivity of catalysts for various products varies. The type and quantity of electrolytes also affect the catalyst's activity and selectivity. While C2 products (such as ethanol, ethylene, and acetic acid) have primarily been observed using copperbased catalysts, C1 products (such as CO, methane, methanol, and formic acid) can develop in a variety of materials [68,69]. Therefore, we primarily concentrated on several electrolytes and catalyst architectures for the conversion of CO2. One of the most critical problems of the electrochemical CO 2 R technologies to function at large-scale is to obtain a great CO 2 selectivity to desired value-added products to reduce product separation costs and complexity. High selectivity is difficult to achieve due to, as shown in Figure 2b, the majority of CO 2 R reactions' standard potentials (Eo) and the hydrogen evolution reaction (HER) all being within a limited variety (−0.250 V to 0.169 V vs. standard hydrogen electrode) (SHE) [36].

Product Selectivity Parameters
The applied potential, pressure, temperature, type of electrolyte (pH, concentration, and composition), and type of electrocatalyst (crystallographic structure, chemical state, composition, and morphology) are all variables that affect selectivity, FE, and ECR performance.
In addition, the selectivity of catalysts for various products varies. The type and quantity of electrolytes also affect the catalyst's activity and selectivity. While C 2 products (such as ethanol, ethylene, and acetic acid) have primarily been observed using copperbased catalysts, C 1 products (such as CO, methane, methanol, and formic acid) can develop in a variety of materials [68,69]. Therefore, we primarily concentrated on several electrolytes and catalyst architectures for the conversion of CO 2 .

Metals
A CO 2 radical anion is produced by electrochemical reduction with an inert metal or carbon electrode. This radical anion can be dimerized to form oxalate or disproportionated to form CO and carbonate. Active metals, on the other hand, may direct CO 2 reduction to hydrogenated products via active sites on their surface at a significantly reduced voltage applied. The metal in these systems performs a dual purpose, supplying electrons while also stabilizing the reduced pieces.
In the 1980s, Hori performed the key study that first established CO 2 RR. In this work, he demonstrated the ability of several pure metal catalysts to reduce CO 2 , which paved the way for extensive CO 2 R research [70]. He demonstrated that pure metal catalysts could be classified into four groups: (1) transition metals that primarily produce CO, such as Au, Ag, Zn, and Pd; (2) main group metals that produce formate (HCOO − ), such as Pb, In, and Sn; (3) metals with negligible activity toward CO2RR, such as Ni, Fe, Pt, and Ti; and (4) Cu, which can produce hydrocarbons and multicarbon products [71].
Monteiro et al. [72] conducted CO 2 R in varying current densities in sulfate electrolytes (100-200 mA cm −2 ) using a 10 cm 2 gold gas-diffusion electrode. According to their findings, moderately acidic media can support high CO selectivity (90%) at 100-200 mA cm −2 , provided insufficiently hydrated cations (Cs + , K + ) are available in the electrolyte. They discovered that in 1 M Cs 2 SO 4 electrolyte, CO 2 R can be performed at notably lower cell potentials than in a neutral medium (1 M KHCO 3 ), resulting in a decrease of up to 30% in process energy expenditures. According to recent findings in idealized small-scale DEMS measurements that proton reduction can be suppressed during CO 2 R under suitable conditions, their results show that FEs of 80-90% for CO can be obtained at a gold gasdiffusion electrode at current densities up to 200 mA/cm 2 , demonstrating the feasibility of running CO 2 electrolysis in acidic media ( Figure 3).

Metals
A CO2 radical anion is produced by electrochemical reduction with an inert metal or carbon electrode. This radical anion can be dimerized to form oxalate or disproportionated to form CO and carbonate. Active metals, on the other hand, may direct CO2 reduction to hydrogenated products via active sites on their surface at a significantly reduced voltage applied. The metal in these systems performs a dual purpose, supplying electrons while also stabilizing the reduced pieces.
In the 1980s, Hori performed the key study that first established CO2RR. In this work, he demonstrated the ability of several pure metal catalysts to reduce CO2, which paved the way for extensive CO2R research [70]. He demonstrated that pure metal catalysts could be classified into four groups: (1) transition metals that primarily produce CO, such as Au, Ag, Zn, and Pd; (2) main group metals that produce formate (HCOO − ), such as Pb, In, and Sn; (3) metals with negligible activity toward CO2RR, such as Ni, Fe, Pt, and Ti; and (4) Cu, which can produce hydrocarbons and multicarbon products [71].
Monteiro et al. [72] conducted CO2R in varying current densities in sulfate electrolytes (100-200 mA cm −2 ) using a 10 cm 2 gold gas-diffusion electrode. According to their findings, moderately acidic media can support high CO selectivity (90%) at 100-200 mA cm −2 , provided insufficiently hydrated cations (Cs + , K + ) are available in the electrolyte. They discovered that in 1 M Cs2SO4 electrolyte, CO2R can be performed at notably lower cell potentials than in a neutral medium (1 M KHCO3), resulting in a decrease of up to 30% in process energy expenditures. According to recent findings in idealized small-scale DEMS measurements that proton reduction can be suppressed during CO2R under suitable conditions, their results show that FEs of 80-90% for CO can be obtained at a gold gasdiffusion electrode at current densities up to 200 mA/cm 2 , demonstrating the feasibility of running CO2 electrolysis in acidic media ( Figure 3).  In a new method described by Wang et al. [73] (Figure 4) the number of active sites on bimetallic catalysts and their size are engineered to increase the efficiency of CO 2 reduction. Pd was added to Au nanoparticles in a variety of regulated dosages to create Pd@Au electrocatalysts. The nonlinear dependency of their catalytic activity for the conversion of CO 2 to CO was attributed to the fluctuation of *CO and *COOH adsorption energies on the Pd sites of various ensemble sizes. In contrast, FE HCOO − grows from 9.8% for Pd5@Au95 to 56% for pure Pd at 0.3 V, whereas J HCOO − rises from 0.019 to 0.059 mA/cm 2 . Bimetallic Pd-Au surfaces with discrete, atomically scattered Pd ensembles have lower energy barriers for CO 2 activation than pure Au and are also less poisoned by strongly binding *CO intermediates than pure Pd, with Pd dimers striking a balance of these two rate-limiting variables.
In a new method described by Wang et al. [73] (Figure 4) the number of active sites on bimetallic catalysts and their size are engineered to increase the efficiency of CO2 reduction. Pd was added to Au nanoparticles in a variety of regulated dosages to create Pd@Au electrocatalysts. The nonlinear dependency of their catalytic activity for the conversion of CO2 to CO was attributed to the fluctuation of *CO and *COOH adsorption energies on the Pd sites of various ensemble sizes. In contrast, FEHCOO-grows from 9.8% for Pd5@Au95 to 56% for pure Pd at 0.3 V, whereas JHCOO-rises from 0.019 to 0.059 mA/cm 2 . Bimetallic Pd-Au surfaces with discrete, atomically scattered Pd ensembles have lower energy barriers for CO2 activation than pure Au and are also less poisoned by strongly binding *CO intermediates than pure Pd, with Pd dimers striking a balance of these two rate-limiting variables.

Metal Oxides
Metal oxides display an extremely wide variety of capabilities due to their various bonding, structures, and compositions. Further, defects in metal oxides give them a range of functions, and the capacity to chemically adjust the distribution, population, and type of defects in the bulk and on the surface of metal oxides provides attraction in many applications [74]. Employing heterostructures using metal and metal oxide is an efficient means of generating new chances for improved catalysis. Strong metal/oxide interactions have been frequently used to enhance the kinetics of chemical catalysis in the gas phase; however, analogous notions for electrocatalysis in the liquid phase have received far less attention [75].
Chen et al. [76] describe the electrochemical creation of CH4 from CO2 conversion. Theoretical studies and experimental results suggest that the Cu species are decreased to metallic Cu on the surface of perovskite and partially exsolved from the perovskite, and the resulting Cu/La2CuO4 heterostructure may be the cause for the efficient CO2 methanation procedure. Methane was produced over this perovskite catalyst at −1.4 VRHE ( Figure  5), With a current density of 117 mA/cm 2 and a FE of 56.3%. This research demonstrates

Metal Oxides
Metal oxides display an extremely wide variety of capabilities due to their various bonding, structures, and compositions. Further, defects in metal oxides give them a range of functions, and the capacity to chemically adjust the distribution, population, and type of defects in the bulk and on the surface of metal oxides provides attraction in many applications [74]. Employing heterostructures using metal and metal oxide is an efficient means of generating new chances for improved catalysis. Strong metal/oxide interactions have been frequently used to enhance the kinetics of chemical catalysis in the gas phase; however, analogous notions for electrocatalysis in the liquid phase have received far less attention [75].
Chen et al. [76] describe the electrochemical creation of CH 4 from CO 2 conversion. Theoretical studies and experimental results suggest that the Cu species are decreased to metallic Cu on the surface of perovskite and partially exsolved from the perovskite, and the resulting Cu/La 2 CuO 4 heterostructure may be the cause for the efficient CO 2 methanation procedure. Methane was produced over this perovskite catalyst at −1.4 VRHE (Figure 5), With a current density of 117 mA/cm 2 and a FE of 56.3%. This research demonstrates an efficient perovskite electrocatalyst for ambient electrochemical CO 2 methanation and may provide a significant understanding of the structural evolution and surface reconstruction of electrocatalytic materials during electrochemical reactions in energy-relevant technologies. an efficient perovskite electrocatalyst for ambient electrochemical CO2 methanation and may provide a significant understanding of the structural evolution and surface reconstruction of electrocatalytic materials during electrochemical reactions in energy-relevant technologies. For the large-scale production of Cu catalysts which are oxide-derived with stable Cu/Cu2O surfaces for highly active CO2RR to C2H4 with high FE and sustained stability, Liu et al. [77] (Figure 6) have developed an anodic oxidation approach. The stable Cu/Cu2O interfaces and high degree of oxidation of the vertically stacked Cu nanoplates on the Cu foil during the CO2RR preclude the clustering of nanostructures. With the help of these properties, the DVL-Cu catalyst obtains high FEC2H4 and EEC2H4 values of 84.5 and 1.7%, 28.9 and 1.3%, and 27.6 and 0.8%, respectively, in the flow cell and 200 mA/cm 2 in the MEA electrolyzer. In fact, the DVL-Cu catalyst keeps the flow-electrolysis cell's performance constant for about 55 h. According to density functional theory (DFT) calculations, the energy barrier for C-C coupling is dramatically lowered because Cu + species increase the *OCCOH intermediate's ability for adsorption. The DVL-Cu catalyst's remarkable selectivity, long-lasting stability, and ease of manufacture indicate its potential for use in achieving the industrial conversion of CO2 to C2H4. For the large-scale production of Cu catalysts which are oxide-derived with stable Cu/Cu 2 O surfaces for highly active CO 2 RR to C 2 H 4 with high FE and sustained stability, Liu et al. [77] (Figure 6) have developed an anodic oxidation approach. The stable Cu/Cu 2 O interfaces and high degree of oxidation of the vertically stacked Cu nanoplates on the Cu foil during the CO 2 RR preclude the clustering of nanostructures. With the help of these properties, the DVL-Cu catalyst obtains high FE C2H4 and EE C2H4 values of 84.5 and 1.7%, 28.9 and 1.3%, and 27.6 and 0.8%, respectively, in the flow cell and 200 mA/cm 2 in the MEA electrolyzer. In fact, the DVL-Cu catalyst keeps the flow-electrolysis cell's performance constant for about 55 h. According to density functional theory (DFT) calculations, the energy barrier for C-C coupling is dramatically lowered because Cu + species increase the *OCCOH intermediate's ability for adsorption. The DVL-Cu catalyst's remarkable selectivity, long-lasting stability, and ease of manufacture indicate its potential for use in achieving the industrial conversion of CO 2 to C 2 H 4 .
Anzai et al. [78] (Figure 7) successfully synthesized Cu-TiO 2 composite catalysts with well-dispersed Cu clusters or nanoparticles using a one-pot solvothermal technique after which the CO 2 is reduced electrochemically through thermal treatment. CuO x clusters were disseminated on the TiO 2 surface in Cu-TiO 2 samples obtained by air calcination of the precursor, while Cu NPs were produced in Cu-TiO 2 -H samples obtained by subjecting the precursor to hydrogen. Cu-TiO 2 -H was discovered to have a good selectivity for CH 4 in electrochemical CO 2 R. At a CH 4 partial current density of 36 mA/cm 2 at −1.8 V RHE , faradaic efficiency for CH 4 synthesis reached 18%. Moreover, 70% of FE CH4 /FE C1+C2 was obtained at −1.8 V RHE . They believe that the homogeneity of the Cu NPs produced on TiO 2 is one of the critical criteria for maximizing CH 4 selectivity in the electrochemical CO 2 R.
1.7%, 28.9 and 1.3%, and 27.6 and 0.8%, respectively, in the flow cell and 200 mA/cm 2 in the MEA electrolyzer. In fact, the DVL-Cu catalyst keeps the flow-electrolysis cell's performance constant for about 55 h. According to density functional theory (DFT) calculations, the energy barrier for C-C coupling is dramatically lowered because Cu + species increase the *OCCOH intermediate's ability for adsorption. The DVL-Cu catalyst's remarkable selectivity, long-lasting stability, and ease of manufacture indicate its potential for use in achieving the industrial conversion of CO2 to C2H4. Anzai et al. [78] (Figure 7) successfully synthesized Cu-TiO2 composite catalysts with well-dispersed Cu clusters or nanoparticles using a one-pot solvothermal technique after which the CO2 is reduced electrochemically through thermal treatment. CuOx clusters were disseminated on the TiO2 surface in Cu-TiO2 samples obtained by air calcination of the precursor, while Cu NPs were produced in Cu-TiO2-H samples obtained by subjecting the precursor to hydrogen. Cu-TiO2-H was discovered to have a good selectivity for CH4 in electrochemical CO2R. At a CH4 partial current density of 36 mA/cm 2 at −1.8 VRHE, faradaic efficiency for CH4 synthesis reached 18%. Moreover, 70% of FECH4/FEC1+C2 was obtained at -1.8 VRHE. They believe that the homogeneity of the Cu NPs produced on TiO2 is one of the critical criteria for maximizing CH4 selectivity in the electrochemical CO2R. Anzai et al. [78] (Figure 7) successfully synthesized Cu-TiO2 composite catalysts with well-dispersed Cu clusters or nanoparticles using a one-pot solvothermal technique after which the CO2 is reduced electrochemically through thermal treatment. CuOx clusters were disseminated on the TiO2 surface in Cu-TiO2 samples obtained by air calcination of the precursor, while Cu NPs were produced in Cu-TiO2-H samples obtained by subjecting the precursor to hydrogen. Cu-TiO2-H was discovered to have a good selectivity for CH4 in electrochemical CO2R. At a CH4 partial current density of 36 mA/cm 2 at −1.8 VRHE, faradaic efficiency for CH4 synthesis reached 18%. Moreover, 70% of FECH4/FEC1+C2 was obtained at -1.8 VRHE. They believe that the homogeneity of the Cu NPs produced on TiO2 is one of the critical criteria for maximizing CH4 selectivity in the electrochemical CO2R.

Two-Dimensional Materials
The most promising method for achieving a carbon-neutral cycle is CO 2 conversion into hydrocarbon fuels, which could be accomplished with the help of advances in electrocatalysis science. The possibility for two-dimensional materials to operate as highly efficient electrocatalytic CO 2 reduction catalysts has recently been recognized [79]. As an illustration, transition-metal dichalcogenides (TMDCs) interacting with ionic liquid (IL) electrolytes can approach a CO 2 reduction system that benefits from materials with overlying of the d-band partial density of states with the Fermi energy, low work function, and an electrolyte "solvent" that effectively transports CO 2 to the active site [80].
Abbasi et al. [81] (Figure 8) produced VA-Mo 1−x M x S 2 structures (M = Nb and Ta) and investigated their efficiency as electrocatalysts in the CO 2 reduction procedure. The maximum catalytic performance was determined to be Va-Mo 0.95 Nb 0.05 S 2 with a CO formation TOF of this structure, which was shown to be dual magnitudes greater than a Ag nanoparticle catalyst over the whole spectrum of overpotentials. The best results were obtained when the dopants in the MoS 2 structure were embedded in the catalysts' atomic structure, implying that this could provide a viable path to enhance the edge atoms of the catalytic performance by altering their electronic characteristics. The influence of Nb in the Mo 1−x M x S 2 structure was investigated using DFT computations. The DFT results for Ta-doped MoS 2 also indicate that Ta doping in the second Mo row of MoS 2 may result in an unfavorable reaction pathway, i.e., the production of COOH * becomes endergonic. Although pure TaS 2 appears to have appropriate reaction pathways, its greater work function (5.5 eV vs. 5.0 eV for MoS 2 ) might be a disadvantage for its electron-transfer property. Thus, unlike Nb-doped MoS 2 , the DFT simulations revealed that Ta-doped MoS 2 is unlikely to have a satisfactory "trade-off" effect between the reaction energetics and the work function.

Two-Dimensional Materials
The most promising method for achieving a carbon-neutral cycle is CO2 conversion into hydrocarbon fuels, which could be accomplished with the help of advances in electrocatalysis science. The possibility for two-dimensional materials to operate as highly efficient electrocatalytic CO2 reduction catalysts has recently been recognized [79]. As an illustration, transition-metal dichalcogenides (TMDCs) interacting with ionic liquid (IL) electrolytes can approach a CO2 reduction system that benefits from materials with overlying of the d-band partial density of states with the Fermi energy, low work function, and an electrolyte "solvent" that effectively transports CO2 to the active site [80].
Abbasi et al. [81] (Figure 8) produced VA-Mo1-xMxS2 structures (M = Nb and Ta) and investigated their efficiency as electrocatalysts in the CO2 reduction procedure. The maximum catalytic performance was determined to be Va-Mo0.95Nb0.05S2 with a CO formation TOF of this structure, which was shown to be dual magnitudes greater than a Ag nanoparticle catalyst over the whole spectrum of overpotentials. The best results were obtained when the dopants in the MoS2 structure were embedded in the catalysts' atomic structure, implying that this could provide a viable path to enhance the edge atoms of the catalytic performance by altering their electronic characteristics. The influence of Nb in the Mo1-xMxS2 structure was investigated using DFT computations. The DFT results for Ta-doped MoS2 also indicate that Ta doping in the second Mo row of MoS2 may result in an unfavorable reaction pathway, i.e., the production of COOH * becomes endergonic. Although pure TaS2 appears to have appropriate reaction pathways, its greater work function (5.5 eV vs. 5.0 eV for MoS2) might be a disadvantage for its electron-transfer property. Thus, unlike Nb-doped MoS2, the DFT simulations revealed that Ta-doped MoS2 is unlikely to have a satisfactory "trade-off" effect between the reaction energetics and the work function. Yang et al. [82] describe that Bismuth nanosheets (BiNSs) are produced via a scalable, facile wet chemical method, and its increased electrocatalytic CO2RR performance toward HCOOsynthesis is demonstrated. Bi-single-atom layers were produced for the first time due to their high atom density, outstanding electrical conductivity, the high surface density of more intrinsically active sites, and improved structural stability. With a high FE of 99%, durability (>75 h), and a low onset overpotential of 90 mV, these layers may selectively catalyze the electroreduction of CO2 to HCOO − exclusively (Figure 9). Their theoretical research suggests that the thickset BiNSs, which expose the (011) facet, firmly bind reaction intermediates, possibly poisoning them. Yang et al. [82] describe that Bismuth nanosheets (BiNSs) are produced via a scalable, facile wet chemical method, and its increased electrocatalytic CO 2 RR performance toward HCOO-synthesis is demonstrated. Bi-single-atom layers were produced for the first time due to their high atom density, outstanding electrical conductivity, the high surface density of more intrinsically active sites, and improved structural stability. With a high FE of 99%, durability (>75 h), and a low onset overpotential of 90 mV, these layers may selectively catalyze the electroreduction of CO 2 to HCOO − exclusively (Figure 9). Their theoretical research suggests that the thickset BiNSs, which expose the (011) facet, firmly bind reaction intermediates, possibly poisoning them.

Functional Microorganisms
Microbial electrosynthesis (MES) uses electrographic microorganisms as biocatalysts to generate compounds from CO2. MES can reduce CO2 using an electroactive bio-film electrode as a catalyst [83][84][85]. Microbial adherence is affected by the range of electrode geometries and material qualities, which impacts biofilm formation and electron exchange [86]. Microbial fuel cells provide many advantages over biomass energy production, including high energy conversion efficiency, room-temperature operation, no requirement for gas treatment, and low energy input [85].
Catalytic CO2 reduction and bioconversion could significantly increase the amount of carbon captured and used while reducing climate change. Current technologies are unfortunately constrained by inefficient electron and mass transfers, poor metabolic kinetics, and a paucity of molecular building blocks. Zhang et al. [86] (Figure 10) overcome these challenges by using electrocatalysis, a chemical-biological (chem-bio) interface, and systematic microbial design to enable efficient electro-microbial conversion with C2 (EMC2). Faster mass transfer, simpler entry into primary metabolism, reduced toxicity, increased energy and electron transport, and superior molecular building blocks for many bacteria are all advantages of soluble C2 intermediates. The EMC2 system's multi-tier chem-bio interface architecture increased microbial biomass productivity by six and eight times in contrast to the C1 intermediate and hydrogen-driven pathways, respectively.

Functional Microorganisms
Microbial electrosynthesis (MES) uses electrographic microorganisms as biocatalysts to generate compounds from CO 2 . MES can reduce CO 2 using an electroactive bio-film electrode as a catalyst [83][84][85]. Microbial adherence is affected by the range of electrode geometries and material qualities, which impacts biofilm formation and electron exchange [86]. Microbial fuel cells provide many advantages over biomass energy production, including high energy conversion efficiency, room-temperature operation, no requirement for gas treatment, and low energy input [85].
Catalytic CO 2 reduction and bioconversion could significantly increase the amount of carbon captured and used while reducing climate change. Current technologies are unfortunately constrained by inefficient electron and mass transfers, poor metabolic kinetics, and a paucity of molecular building blocks. Zhang et al. [86] (Figure 10) overcome these challenges by using electrocatalysis, a chemical-biological (chem-bio) interface, and systematic microbial design to enable efficient electro-microbial conversion with C2 (EMC2). Faster mass transfer, simpler entry into primary metabolism, reduced toxicity, increased energy and electron transport, and superior molecular building blocks for many bacteria are all advantages of soluble C2 intermediates. The EMC2 system's multi-tier chem-bio interface architecture increased microbial biomass productivity by six and eight times in contrast to the C1 intermediate and hydrogen-driven pathways, respectively. challenges by using electrocatalysis, a chemical-biological (chem-bio) interface, and systematic microbial design to enable efficient electro-microbial conversion with C2 (EMC2). Faster mass transfer, simpler entry into primary metabolism, reduced toxicity, increased energy and electron transport, and superior molecular building blocks for many bacteria are all advantages of soluble C2 intermediates. The EMC2 system's multi-tier chem-bio interface architecture increased microbial biomass productivity by six and eight times in contrast to the C1 intermediate and hydrogen-driven pathways, respectively. C. scatologenes is an acetogenic bacterium that can fix CO2 via the Wood-Ljungdahl route and operate as a biocatalyst in MES systems, as described by Liu et al. [87] (Figure  11). At a potential of −0.6 V, the cathodic chamber produced the highest amounts of acetic acid and butyric acid, 0.03 and 0.01 g/L, respectively. The maximum total coulombic efficiency was approximately 84%. The LES system adopted H2 when the cathodic potential decreased and ethanol was found in the product spectrum. Nonetheless, due to the reduced H2 evolution rate, the MES system's H2 usage rate fell to 37.8%. Overall, the synthesis of butyric acid, a C-4 molecule, increases the possibility for MES deployment greatly. Based on genome sequencing, direct electron transfer (DET) in the MES system is thought to be aided by hydrogenase and ATPase. Furthermore, C. scatologenes, like C. ljungdahlii and C. aceticum, should theoretically be capable of accepting electrons straight from an electrode. C. scatologenes is an acetogenic bacterium that can fix CO 2 via the Wood-Ljungdahl route and operate as a biocatalyst in MES systems, as described by Liu et al. [87] (Figure 11). At a potential of −0.6 V, the cathodic chamber produced the highest amounts of acetic acid and butyric acid, 0.03 and 0.01 g/L, respectively. The maximum total coulombic efficiency was approximately 84%. The LES system adopted H 2 when the cathodic potential decreased and ethanol was found in the product spectrum. Nonetheless, due to the reduced H 2 evolution rate, the MES system's H 2 usage rate fell to 37.8%. Overall, the synthesis of butyric acid, a C-4 molecule, increases the possibility for MES deployment greatly. Based on genome sequencing, direct electron transfer (DET) in the MES system is thought to be aided by hydrogenase and ATPase. Furthermore, C. scatologenes, like C. ljungdahlii and C. aceticum, should theoretically be capable of accepting electrons straight from an electrode. duced H2 evolution rate, the MES system's H2 usage rate fell to 37.8%. Overall, the synthesis of butyric acid, a C-4 molecule, increases the possibility for MES deployment greatly. Based on genome sequencing, direct electron transfer (DET) in the MES system is thought to be aided by hydrogenase and ATPase. Furthermore, C. scatologenes, like C. ljungdahlii and C. aceticum, should theoretically be capable of accepting electrons straight from an electrode. Chatzipanagiotou et al. [88] (Figure 12) demonstrated that metabolic cooperation between copper electrocatalysts and MES biocatalysts, with formate as a metabolic intermediary, is possible. Up to 140 mg L −1 of formate was created entirely by copper oxide, indicating a co-catalytic (i.e., metabolic) interaction, whereas formate was also clearly formed Chatzipanagiotou et al. [88] (Figure 12) demonstrated that metabolic cooperation between copper electrocatalysts and MES biocatalysts, with formate as a metabolic intermediary, is possible. Up to 140 mg L −1 of formate was created entirely by copper oxide, indicating a co-catalytic (i.e., metabolic) interaction, whereas formate was also clearly formed by copper and eaten by bacteria generating acetate. The two catalysts had syntrophic effects, with CuOx electrodes resulting in a threefold increase in current density and acetate generation. Although biofilm coverage on the electrode's surface was not explored, it was shown that the presence of microorganisms in the electrolyte had no effect on copper's long-term catalytic activity for CO 2 reduction to formate. Methods for improving the performance of co-catalytic ideas based on copper catalysts are reviewed, such as modifying the electrode shape and electrolyte composition. by copper and eaten by bacteria generating acetate. The two catalysts had syntrophic effects, with CuOx electrodes resulting in a threefold increase in current density and acetate generation. Although biofilm coverage on the electrode's surface was not explored, it was shown that the presence of microorganisms in the electrolyte had no effect on copper's long-term catalytic activity for CO2 reduction to formate. Methods for improving the performance of co-catalytic ideas based on copper catalysts are reviewed, such as modifying the electrode shape and electrolyte composition. The connection between a metal catalyst (for example, iron, shown as grey nanoparticles) and an MES culture is depicted schematically (illustrated as orange cells) [88].
Here, the summary of electrocatalytic CO2 conversion in various electrode structures with their optimal condition is highlighted in Table 2. Table 2 shows the copper and copper oxide, copper-oxide-derived, copper-carbon catalysts, and doped copper catalysts. The dominant usage of copper can be explained by its ability to produce multicarbon products during the reduction of CO2. The use of Cu electrodes in CO2 reduction experiments allows for the formation of a broad variety of products. Thereby, it becomes visible that the selective formation of multicarbon alcohols still possesses a challenge. Ethylene is generally preferred to ethanol formation in copper-based electrodes. As can also be seen from Table 2, the most frequently used electrolytes are KHCO3 and KOH. One key parameter with a strong influence on catalyst/electrode performance is the electrolyte. Compared to KHCO3, a higher selectivity for carbonaceous products using KOH was shown. High local pH values, which can be favored by an electrolyte with low buffer capacity, have been shown to improve the product distribution toward higher hydrocarbons. Even when comparing 1 M KOH with 1 M or 0.1 M KHCO3, clear differences can already be seen. Although the same current densities can be achieved in principle with both electrolytes, the same current densities can be reached with 1 M KOH at considerably lower voltages; because the CO2RR activity is significantly higher there in a catholyte with higher The connection between a metal catalyst (for example, iron, shown as grey nanoparticles) and an MES culture is depicted schematically (illustrated as orange cells) [88].
Here, the summary of electrocatalytic CO 2 conversion in various electrode structures with their optimal condition is highlighted in Table 2. Table 2 shows the copper and copper oxide, copper-oxide-derived, copper-carbon catalysts, and doped copper catalysts. The dominant usage of copper can be explained by its ability to produce multicarbon products during the reduction of CO 2 . The use of Cu electrodes in CO 2 reduction experiments allows for the formation of a broad variety of products. Thereby, it becomes visible that the selective formation of multicarbon alcohols still possesses a challenge. Ethylene is generally preferred to ethanol formation in copper-based electrodes. As can also be seen from Table 2, the most frequently used electrolytes are KHCO 3 and KOH. One key parameter with a strong influence on catalyst/electrode performance is the electrolyte. Compared to KHCO 3 , a higher selectivity for carbonaceous products using KOH was shown. High local pH values, which can be favored by an electrolyte with low buffer capacity, have been shown to improve the product distribution toward higher hydrocarbons. Even when comparing 1 M KOH with 1 M or 0.1 M KHCO 3 , clear differences can already be seen. Although the same current densities can be achieved in principle with both electrolytes, the same current densities can be reached with 1 M KOH at considerably lower voltages; because the CO 2 RR activity is significantly higher there in a catholyte with higher basicity, less energy is therefore required for the CO 2 RR. In addition, the use of 1 M KOH also shifts the selectivity toward carbonaceous products.

Summary and Perspectives
In the last decade, significant progress in CO 2 RR has been made by analyzing reaction mechanisms and creating electrocatalysts, electrolytes, and electrolyzes. In this work, we reviewed recent reports on the building of improved and efficient electrocatalytic reactions for the CO 2 conversion to value-added commodities using engineering approaches.
To improve electrocatalytic reactions for converting CO 2 to value-added products, we found that there is still a dearth of knowledge, which adds to the remaining challenges. CO 2 RR based on modified metal, metal oxide, and two-dimensional materials may be viable electrocatalyst materials for converting CO 2 to commercially valuable compounds. Sustainable energy production using electrochemical CO 2 conversion with low environmental impact offers an excellent opportunity to reduce fossil fuel consumption.
The development of new catalyst materials and composites with computational modeling and mechanistic studies should be the focus of future research to create stable and highly effective electrocatalysts. (1) Now, a major issue for the CO 2 RR is salt precipitation brought on by carbonate formation on the cathode side, which lowers the catalyst surface's active area. (2) There are issues about the stability and scalability of CO 2 RR research because new-structured devices can increase CO 2 conversion efficiency and support stability.
(3) The CO 2 RR aims to treat CO 2 emissions from fossil fuel combustion in real-world scenarios. The requirement for flue gas CO 2 capture is eliminated by directly substituting feed gas with flue gas from the combustion of fossil fuels. This process necessitates a large amount of energy as well as additional purification units.